Lithography system, control system and device manufacturing method

ABSTRACT

A lithography system in which a performance criterion of the lithography system is predicted, based on one or more operating conditions of the lithography system, and compared to measurements of that performance criterion. The lithography system may determine from a difference between the measured and predicted performance criterion which, if any, sub-system of the lithography system is not performing as expected.

FIELD

The present invention relates to a lithography system, its controlsystem and to a method of manufacturing devices using a lithographictechnique, in particular employing a method of inspection usable, forexample, in the manufacture of devices by a lithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is necessary to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. There are varioustechniques for making measurements of the microscopic structures formedin a lithographic process, including the use of a scanning electronmicroscope and various specialized tools. One form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known. Aspectroscopic scatterometer directs a broadband radiation beam onto thesubstrate and measures the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. An angularly resolved scatterometer uses a monochromaticradiation beam and measures the intensity of the scattered radiation asa function of angle.

Scatterometry is an active field of research where optical techniquesare used to measure subwavelength features of an object. An embodimentof the system of the present invention may be used with apparatusconfigured to measure the subwavelength features, such as an in-linemetrology tool. Such a metrology tool detects a reflected beam that hasbeen reflected from the surface of a substrate and more specificallyfrom a specific target on the substrate, and from the reflected beam andits different diffraction orders, reconstructs the shape of the targeton the substrate.

As manufacturing margins for the formation of, for example, integratedcircuit devices become smaller and the complexity of lithographicapparatus becomes ever greater, it becomes more significant to monitorthe performance of the equipment used in the various parts of thelithography process. Typically this involves executing dedicated testsfor each sub-system or performance parameter. However, these tests arecostly, time-consuming, do not always address on-product performancecriterion and/or are often not very specific in identifying root-causesin errors that are detected. Furthermore, it is often the case that bythe time such a test has been completed and the results analyzed, thetested sub-system will have processed many substrates in the whatevercondition it is in. Accordingly, a large number of substrates may beprocessed by a faulty sub-system before the fault is detected, resultingin large numbers of substrates either needing re-work or, in some cases,scrapping. Clearly this may be very costly.

SUMMARY

It is desirable, for example, to provide a system in which a deviationfrom a desired performance level of a sub-system within a lithographysystem can be more easily identified.

According to an aspect of the invention, there is provided a lithographysystem comprising:

a performance prediction unit, configured to predict a performancecriterion of the formation of a pattern on a substrate by thelithography system based on an operating condition of the lithographysystem;

an exposure unit, configured to expose a pattern of radiation on thesubstrate;

a performance measurement unit, configured to measure a performancecriterion of the formation of the pattern on the substrate by thelithography system, the measured performance criterion corresponding tothe predicted performance criterion;

a comparison unit, configured to compare the predicted performancecriterion with the corresponding measured performance criterion; and

a process controller, configured to control the lithography system,

wherein subsequent control of the lithography system is regulated by thedifference in the predicted and measured performance criteriondetermined by the comparison unit.

According to an aspect of the invention, there is provided a method ofmanufacturing a device using a lithography system comprising:

predicting a performance criterion of the formation of a pattern on asubstrate by the lithography system, based on an operating condition ofthe lithography system;

exposing a pattern of radiation on the substrate;

measuring a performance criterion of the formation of the pattern on thesubstrate by the lithography system, the measured performance criterioncorresponding to the predicted performance criterion; and

comparing the predicted performance criterion with the correspondingmeasured performance criterion,

wherein the subsequent control of the lithography system is regulated bythe difference in the predicted and measured performance criterion.

According to an aspect of the invention, there is provided a computerprogram for controlling a lithography system comprising:

a performance prediction section, configured to predict a performancecriterion of the formation of a pattern on a substrate by thelithography system based on an operating condition of the lithographysystem;

a performance measurement section, configured to measure a performancecriterion of the formation of the pattern on the substrate by thelithography system, the measured performance criterion corresponding tothe predicted performance criterion;

a comparison section, configured to compare the predicted performancecriterion with the corresponding measured performance criterion; and

a process control section, configured to control the lithography system,

wherein the subsequent control of the lithography system by the processcontrol section is regulated by the difference in the predicted andmeasured performance criterion determined by the comparison section.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 a depicts a lithographic apparatus;

FIG. 1 b depicts a lithographic cell or cluster;

FIG. 2 depicts a first scatterometer;

FIG. 3 depicts a second scatterometer; and

FIG. 4 depicts a lithography system according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 a schematically depicts a lithographic apparatus. The apparatuscomprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1 a, the illuminator IL receives a radiation beam froma radiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PL,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder, 2D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1 a) can be used to accuratelyposition the patterning device MA with respect to the path of theradiation beam B, e.g. after mechanical retrieval from a mask library,or during a scan. In general, movement of the support structure MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PL. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 1 b, the lithographic apparatus LA, also referred to asan exposure unit, forms part of a lithographic cell LC, also sometimesreferred to a lithocell or cluster, which also includes apparatus toperform pre- and/or post-exposure processes on a substrate.Conventionally such apparatus may include one or more spin coaters SC todeposit resist layers, one or more developers DE to develop exposedresist, one or more chill plates CH and one or more bake plates BK. Asubstrate handler, or robot, RO, which may also be part of thelithographic cell picks up substrates from input/output ports I/O1,I/O2, moves them between the different process devices and delivers themto the loading bay LB of the lithographic apparatus. These devices,which are often collectively referred to as the track, are under thecontrol of a track control unit TCU which itself may be controlled by asupervisory control system SCS, which may also control the lithographicapparatus LA. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In general all of the apparatus used to form a device on the substratemay be referred to as a lithography system. It will be appreciated thatthis will include one or more lithographic apparatus, units to performpre- and/or post-exposure processing of the substrate, at least some ofwhich may be associated with a lithographic apparatus to form alithographic cell such as the track described above and optionallyincluding, for example, apparatus to perform etching steps anddeposition steps, and apparatus to inspect substrates and featuresformed on the substrates.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, an adjustment may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that one or more other substrates of the same batch arestill to be exposed. Also, one or more already exposed substrates may bestripped and reworked—to improve yield—or discarded—thereby avoidingperforming exposure on a substrate known to be faulty. In a case whereonly one or some target portions of a substrate are faulty, furtherexposures can be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on an exposed substrate andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 2 depicts a scatterometer which may be used in an embodiment of thepresent invention. It comprises a broadband (white light) radiationprojector 2 which projects radiation onto a substrate W. The reflectedradiation is passed to a spectrometer detector 4, which measures aspectrum 10 (intensity as a function of wavelength) of the specularreflected radiation. From this data, the structure or profile givingrise to the detected spectrum may be reconstructed by processing unitPU, e.g. by Rigorous Coupled Wave Analysis and non-linear regression orby comparison with a library of simulated spectra as shown at the bottomof FIG. 2. In general, for the reconstruction the general form of thestructure is known and some parameters are assumed from knowledge of theprocess by which the structure was made, leaving only a few parametersof the structure to be determined from the scatterometry data. Such ascatterometer may be configured as a normal-incidence scatterometer oran oblique-incidence scatterometer.

Another scatterometer that may be used with an embodiment of the presentinvention is shown in FIG. 3. In this device, the radiation emitted byradiation source 2 is focused using lens system 12 through interferencefilter 13 and polarizer 17, reflected by partially reflective surface 16and is focused onto substrate W via a microscope objective lens 15,which has a high numerical aperture (NA), for example, at least 0.9 orat least 0.95. Immersion scatterometers may even have lenses withnumerical apertures over 1. The reflected radiation then transmitsthrough partially reflective surface 16 to a detector 18 in order tohave the scatter spectrum detected. The detector may be located in theback-projected pupil plane 11, which is at the focal length of the lenssystem 15, however the pupil plane may instead be re-imaged withauxiliary optics (not shown) to the detector. The pupil plane is theplane in which the radial position of radiation defines the angle ofincidence and the angular position defines the azimuth angle of theradiation. In an embodiment, the detector is a two-dimensional detectorso that a two-dimensional angular scatter spectrum of the substratetarget can be measured. The detector 18 may be, for example, an array ofCCD or CMOS sensors, and may use an integration time of, for example, 40milliseconds per frame.

A reference beam is often used, for example, to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the partially reflective surface 16 part of it is transmitted throughthe partially reflective surface 16 as a reference beam towards areference mirror 14. The reference beam is then projected onto adifferent part of the same detector 18.

A set of interference filters 13 is available to select a wavelength ofinterest in the range of, say, 405-790 nm or even lower, such as 200-300nm. The interference filter may be tunable rather than comprising a setof different filters. A grating could be used instead of one or moreinterference filters.

The detector 18 may measure the intensity of scattered radiation at asingle wavelength (or narrow wavelength range), separately at multiplewavelengths or integrated over a wavelength range. Furthermore, thedetector may separately measure the intensity of transverse magnetic-and transverse electric-polarized radiation and/or the phase differencebetween the transverse magnetic- and transverse electric-polarizedradiation.

Using a broadband radiation source (i.e. one with a wide range ofradiation frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband preferablyeach has a bandwidth of *8 and a spacing of at least 2*8 (i.e. twice thewavelength). Several “sources” of radiation can be different portions ofan extended radiation source which have been split using fiber bundles.In this way, angle resolved scatter spectra can be measured at multiplewavelengths in parallel. A 3-D spectrum (wavelength and two differentangles) can be measured, which contains more information than a 2-Dspectrum. This allows more information to be measured which increasesmetrology process robustness. This is described in more detail inEuropean patent application publication no EP 1628164A.

The target on substrate W may be a grating which is printed such thatafter development, the bars are formed of solid resist lines. The barsmay alternatively be etched into the substrate. This pattern issensitive to chromatic aberrations in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberrations will manifest themselvesin a variation in the printed grating. Accordingly, the scatterometrydata of the printed gratings is used to reconstruct the gratings. Theparameters of the grating, such as line widths and shapes, may be inputto the reconstruction process, performed by processing unit PU, fromknowledge of the printing step and/or other scatterometry processes.

The purpose of rigorous optical diffraction theories in featuremetrology is effectively the reconstruction of a diffraction spectrumthat is reflected from the target or mark. In other words, target shapeinformation is obtained for CD (critical dimension) uniformity andoverlay metrology. Overlay metrology is a measuring system in which theoverlay of two marks is measured in order to determine whether twolayers on a substrate are aligned or not. CD uniformity is a measurementof the uniformity of the mark on the spectrum to determine how theexposure system of the lithographic apparatus is functioning.Specifically, CD, or critical dimension, is the width of the smallestobject to be “written” on the substrate and it is important because itis a limit at which a lithographic apparatus is physically able to writeon a substrate.

The way the measurement of the target shape (or “mark shape”) may becarried out is as follows:

1. The target shape is estimated. This estimated shape is givendifferent parameters such as α⁽⁰⁾, β⁽⁰⁾, χ⁽⁰⁾, and so on. Theseparameters may be, for example, respectively the angle of each sidewall, the height of the top of the mark, the width at the top of themark, the width at the bottom of the mark, etc.

2. A rigorous optical diffraction method such as RCWA is used to obtainthe estimated or model diffraction pattern of the estimated targetshape.

3. The diffraction pattern of the actual target on the substrate is thenmeasured by illuminating the target on the substrate with a radiationbeam and detecting the diffracted beam, the pattern of which will bedependent on the properties of the target. This diffraction pattern andthe model diffraction pattern are forwarded to a calculation system suchas a computer.

4. The actual diffraction pattern and the model diffraction pattern arethen compared. Each of the shape parameters is compared and anydifferences are fed into a “merit function” calculation.

5. Using the merit function, which relates the sensitivity of markparameters to the shape of the diffraction pattern, new shape parametersare estimated.

The computation time of this iterative process is largely determined bythe forward diffraction model, i.e. the calculation of the estimateddiffraction model using a rigorous optical diffraction theory from theestimated mark shape. The rest is comparison and less calculation isrequired than would be required using a rigorous model to determine theshape directly from the measured diffraction pattern.

A lithography system according to an embodiment of the present inventionis depicted in FIG. 4. As shown, the lithography system includes apre-exposure processing unit 20 to process a substrate prior toexposure, an exposure unit 21 to expose a pattern on the substrate and apost-exposure processing unit 22 to process a substrate after thepattern is exposed on the substrate. The lithography system furtherincludes a system controller 25 that controls at least the exposure unit21 and may control other parts of the lithography system, including thepre-exposure processing unit 20 and the post-exposure processing unit22. It should be appreciated that although, as depicted in FIG. 4, thecontrol system for the lithography system may be provided at a centralresource, it may also be distributed among the components of thelithography system and/or may be provided separate from the lithographysystem.

The lithography system includes a memory 26 containing the operatingconditions under which the lithography system operates. The systemcontroller 25 may control the lithography system based on the operatingconditions stored in the memory 26. Alternatively, the operatingconditions of the lithography system may be set externally, in whichcase the operating conditions may be provided to the memory 26, forexample either by the external source that provides them to the systemcontroller 25 or by the system controller itself.

The lithography system further includes a performance prediction unit 27that predicts at least one performance criterion of the formation of apattern on the substrate based on one or more of the operatingconditions stored in the memory 26. For example, the performanceprediction unit 27 may include a mathematical model of the processesperformed by the lithography system, from which it can calculate theexpected performance of the lithography system operating under theoperating conditions stored in the memory 26. Alternatively oradditionally, the performance prediction unit 27 may be able tocalculate an expected performance of the lithography system based onhistorical data of the performance of the lithography system previouslywith similar or corresponding operating conditions.

As depicted in FIG. 4, the performance prediction unit 27 may be aseparate component of the lithography system from the system controller25. Alternatively, the performance prediction unit may be part of, orintegrated with, the system controller 25. Similarly, the performanceprediction unit 27 may be provided externally to the lithography systemand the predicted performance criterion may be provided to thelithography system, for example together with the operating conditionsto be used.

The lithography system further includes a substrate inspection device 30and a performance measurement unit 31 that, based on the inspection bythe substrate inspection device 30, determines at least one performancecriterion of the formation of the pattern on the substrate by thelithography system that correspond to the performance criterionpredicted by the performance prediction unit 27. The substrateinspection device 30 and the performance measurement unit 31 may, forexample, be a system based on scatterometry as described above. However,it should be appreciated that any system configured to measure theperformance of the formation of the pattern on the substrate by thelithography system may be used.

Furthermore, it will be appreciated that, although the performanceprediction unit 27 may determine the expected performance of thelithography system prior to the formation of the pattern on thesubstrate, or prior to the measurement by the substrate inspectiondevice 30, or the determination by the performance measurement unit 31,the predicted performance may be determined at the same time as themeasured performance or even afterwards. Likewise, although as depictedin FIG. 4, the substrate inspection device 30 may inspect the substrateafter the substrate has been processed by the post-exposure processingunit 22, the performance measurement of the system may be based onmeasurements made immediately before or after the exposure of thepattern on the substrate.

The lithography system further includes a comparator 35 that determinesthe difference between the predicted performance criterion and thecorresponding measured performance criterion. If the lithography systemis performing as expected, for example according to the model that maybe used in the performance prediction unit 27, one would expect there tobe no difference between the predicted performance criterion and themeasured performance criterion. Accordingly, any difference detected bythe comparator 35 represents a deviation of the performance of thelithography system from the expected performance. As explained below,the system controller 25 may use this information in order to regulatethe subsequent operation of the lithography system. Possible uses ofthis information are set out below as separate embodiments. It should beappreciated, however, that a lithography system according to anembodiment of the present invention may employ any combination of theseembodiments or one or more aspects of these embodiments. As will beappreciated, more than one performance criterion may be predicted,measured and/or compared.

EMBODIMENT 1

As discussed above, a lithography system according to an embodiment ofthe present invention compares one or more predicted and measuredperformance criterion of the formation of a pattern on a substrate bythe lithography system. The system controller 25, for example, may beconfigured such that if the difference between the predicted performancecriterion and the measured performance criterion exceeds a giventhreshold, the operation of the lithography system is suspended. Thismay be beneficial because such a difference may suggest that thelithography system is operating in an unexpected manner. This may resultin a pattern being formed on the substrate that includes errors.Accordingly, suspending the operation of the lithography system at thisstage may prevent a pattern being erroneously formed on the substrate,thus reducing the need for re-work or scrapping of one or moresubstrates processed by the lithography system.

It will be appreciated that, where more than one performance criterionis being monitored, different thresholds may be set for each performancecriterion. Likewise, for example, the operation of the lithographysystem may be suspended if the difference between the predicted andmeasured performance for two or more performance criterion both exceed asecond, lower, negative threshold. Other combinations of thresholds mayalso be provided.

Alternatively or additionally, a lithography system according to thefirst embodiment may include a memory to store previously determineddifferences between the predicted and measured performance criterion,for example within the comparator 35. Accordingly, the rate of change ofthe differences between the predicted and measured performance criterionmay also be monitored.

The lithography system may be configured such that, if the rate ofchange of the difference between a predicted and measured performancecriterion exceeds a given threshold, the operation of the lithographysystem may be suspended. As before, different thresholds may be set fordifference performance criterion or, for example, the operation of thelithography system may be suspended if the rate of change of thedifference between the predicted and measured performance criterion fortwo or more performance criterion both exceed a second, lower,respective threshold. Likewise other combinations of thresholds and/orcombinations of absolute thresholds relating to the difference andthresholds relating to the rate of change of the difference may also beused. Suspending the operation of the lithography system due to anexcessive rate of change of the difference between the predicted andmeasured performance criterion may be beneficial because it may suggestthat the lithography system has become unstable.

EMBODIMENT 2

In a lithography system according to the second embodiment, the systemcontroller 25 is configured to recognize one or more characteristicpatterns in the performance criterion data, as discussed in more detailbelow. Such a characteristic pattern may, for example, correspond to theperformance of one or more individual sub-systems within the lithographysystem. Accordingly, by analysis of the characteristic pattern, thesystem controller may be able to determine the level of performance ofone or more of the sub-systems.

Examples of such sub-systems include the illuminator, the projectionsystem and/or the mask or substrate tables of the exposure unit, atransport unit and/or component within the pre- and post-exposureprocessing units, such as a bake plate to heat substrates, a chill plateto cool substrates, a spin-coater, for example, to apply resist, and/ora developer to develop exposed resist. The sub-systems may also relateto one or more parts of the lithography system not included in alithocell, such as, for example, an etcher and/or polisher.

If a level of performance of one of the sub-systems, as determined bythe system controller from a characteristic pattern of differencesbetween the predicted and measured performance criterion, is beyond agiven threshold (for example, the performance is not sufficient), theoperation of the lithography system may be suspended. It will beappreciated that the threshold performance level for one or moresub-systems may be different. Likewise, as before, the operation of thelithography system may be suspended, for example, if the performance oftwo or more sub-systems is beyond a second, different, respectivethreshold or any combination of thresholds is passed. Likewise, the rateof change of performance of one or more sub-systems may be monitored,for example by the system controller 25 or the comparator 35 and theoperation of the lithography system may be suspended if the rate ofchange of performance of a sub-system exceeds a certain threshold, ifthe rate of change of performance of two or more of the sub-systemsexceeds a second, lower, respective threshold or if any combination ofabsolute performance thresholds and rate of change of performancethresholds is exceeded. As with the first embodiment, an excessive rateof change of the performance of one or more of the sub-systems mayindicate that the lithography system is unstable.

A lithography system may include two or more sub-systems of a particularkind. For example, a lithocell may include more than one of one or moreselected from a chiller plate, bake plate, spin-coater or developermodule. Likewise, a lithography system is likely to include a pluralityof lithocells. Therefore, the lithography system may include a substraterouting memory 40 that records which of the sub-systems is used toperform each of the processes on a given substrate. Accordingly, byusing the information in the substrate routing memory 40 it is possibleto determine which sub-system performed a given process on a substrateand therefore to associate any sub-system performance informationderived from a given substrate with the appropriate sub-system. If it isdetermined that a particular sub-system of one kind is not performingsatisfactorily, the operation of that sub-system may be suspended.However, if the lithography system includes one or more othersub-systems of that type, the operation of the lithography system as awhole need not be suspended because a substrate can be routed to theother sub-system(s) of that type.

It should be appreciated that the substrate routing memory 40 may notonly include historical data relating to the routing of one or moresubstrates but may also include routing information for one or moresubstrates that are yet to be processed. Accordingly, the systemcontroller 25 may use the information from the substrate routing memory40 to control the routing of one or more substrates. Accordingly, if asub-system is suspended from operation, the system controller 25 mayadjust the data in the substrate routing memory 40 for one or moresubsequent substrates to be processed in order to route thatsubstrate(s) to one or more other sub-systems of the same kind.

EMBODIMENT 3

A lithography system according to the third embodiment may be arrangedsimilarly to the second embodiment such that it can determine theperformance of one or more of the sub-systems of the lithography system.However, alternatively or additionally to suspending the operation ofthe lithography system or a sub-system of the lithography system, thelithography system of the third embodiment may be configured such that,when the performance of a sub-system is beyond a given threshold or therate of change of the performance of the sub-system exceeds a giventhreshold, corrective action is taken. For example, the systemcontroller may be configured in such a circumstance to schedule one ormore selected from maintenance, repair, replacement (where it ispossible) or calibration of the sub-system. In particular, thelithography system may be configured such that, not only can it bedetermined which sub-system is not performing in the desired manner butit can also determine from the characteristic pattern of the differencesbetween the predicted and measured performance criterion the nature ofthe fault within the sub-system. The lithography system mayalternatively or additionally be arranged to notify an operator of thesystem, for example such that the operator can investigate thesub-system and determine whether or not any further corrective action isrequired.

EMBODIMENT 4

As with the second and third embodiments, a lithography system accordingto the fourth embodiment is arranged to determine the level ofperformance of one or more of the sub-systems of the lithography systemfrom one or more characteristic patterns of the differences between thepredicted and measured performance criterion for the formation of apattern on a substrate. In addition, the fourth embodiment includes asub-system performance memory 45 that records the performance history ofone or more of the sub-systems. This may be useful, especially for oneor more critical sub-systems, where verification of the performance ofthe lithography system may be required. It may also be useful, where afault does occur, for identifying the nature and/or cause of any suchfault.

EMBODIMENT 5

The system controller 25 of a lithography system according to the fifthembodiment may be configured such that, from the sub-system performancemeasurements determined in the same manner as any of the second tofourth embodiments, the system controller can determine an optimumrouting of a substrate through the lithography system in order toprovide the best possible or at least improved performance of thelithography system for the formation of a pattern on that substrate. Forexample, for each kind of sub-system required to process the substrate,the system controller may select the sub-system within the lithographysystem that is achieving the best level of performance.

Alternatively or additionally, the system controller may be configuredsuch that it can select one of a first kind of sub-system and another ofa second kind of sub-system such that the deviation of one of thesub-systems from its desired performance compensates for the deviationof the other of the sub-systems from its desired performance. In otherwords, instead of selecting the best performing sub-system from eachkind, the system controller may select the best combination ofsub-systems. It should be appreciated that the identification ofsub-systems that, when used together, provide optimum performance is notlimited to pairs of sub-systems. Groups of three or more sub-systemsthat provide optimum performance when combined may be identified.

Such an arrangement as provided by the fifth embodiment may beespecially beneficial because the required performance level of thelithography system may vary between jobs. Accordingly, it may be desiredto select an optimum performing route for the formation of a pattern onsubstrates that require the best possible performance level and use aless highly performing route for the formation of a pattern onsubstrates for which the performance is not critical.

Alternatively or additionally, the system controller may be configuredsuch that one or more performance criterion are optimized, possibly atthe cost of others. For example, the CD and/or overlay performance maybe optimized at the cost of productivity performance. As a furtherexample, productivity performance may be maximized subject toperformance criterion such as CD and/or overlay reaching a minimumrequirement.

EMBODIMENT 6

As with the fifth embodiment, a lithography system according to thesixth embodiment may be configured such that it can determine the levelof performance of each of the sub-systems of the lithography system andcan determine the overall performance of the lithography system for asubstrate processed by a given combination of such sub-systems. Thelithography system may be further configured such that as well as, orinstead of, determining the optimum route for a substrate in order toprovide the best performance of the lithography system, it can determineone or more routes that merely provide sufficient performance capabilityto form a particular pattern on a substrate. Accordingly, a routeyielding better performance of the lithography system may be reservedfor the formation of a pattern on substrates where better performance ofthe lithography system is required.

The lithography system may include a pattern analyzer, for examplewithin the system controller 25, that analyzes a pattern to be formed onthe substrate and determines a level of performance that may be requiredto form the pattern on the substrate (from which the system controllermay determine the appropriate route through the lithography system forthe substrate).

EMBODIMENT 7

A lithography system according to the seventh embodiment may beconfigured such that, as with previous embodiments, it can determine theperformance of one or more sub-systems within the lithography systemfollowing the measurement of one or more performance criterion for theformation of a pattern of a substrate. Accordingly, the systemcontroller 25 of the seventh embodiment may, from the performance of oneor more of the sub-systems, determine whether or not that performance ofthe formation of the pattern on that substrate has been satisfactory. Ifthe performance has not been satisfactory, the system controller maydivert the routing of the substrate such that the substrate may be (atleast partially) re-worked or, if necessary, scrapped. Spotting that theprocessing of the substrate has been unsatisfactory at an early stagemay prevent additional work being performed on a substrate that willsubsequently have to be re-done or scrapped.

EMBODIMENT 8

As with the seventh embodiment, a system controller 25 of a lithographysystem according to the eighth embodiment monitors the performance ofthe pattern formation on the substrate and may determine whether or notthe performance of the pattern formation on a substrate is satisfactory.The lithography system further includes a substrate pattern formationperformance memory 50 that records, for each substrate, the performanceof the processes carried out on each substrate. This may be beneficialbecause, for some devices that are formed on substrates, such asmicroprocessors, although the nominal pattern formed on the substratemay be the same for two different classes of the device, the quality ofthe performance of the processes used to form the device may determinewhether or not the device is in the class having a higher performance ofthe device or a class having a lower performance of the device. Clearly,the higher the performance of the device, the more value it has. Bymonitoring the performance of the processes by which the device isformed, the system controller 25 of the eighth embodiment may identifysubstrates for which, as a result of one or more previous processesbeing formed at a lower performance level, the device being formed willonly be able to attain the given performance level that is lower thanoptimum. In such a situation, it may not be worthwhile forming theremainder of the device at the highest quality because of the limitationon the quality of the device imposed by the relatively low performancelevel of the already completed process(es).

Therefore, from the substrate pattern formation performance memory 50,the system controller 25 may determine for each substrate the requiredlevel of performance of the lithography system at which to perform oneor more subsequent processes on the substrate at the level necessary toproduce a device having the maximum remaining possible performancelevel, given the performance of one or more previous processes.

It should be appreciated that the system controller may be configuredsuch that if part of a substrate can be processed at a higherperformance level than the remainder of the substrate, the systemcontroller selects one or more sub-systems to process the substrate suchthat at least that part of the substrate continues to be processed atthe higher performance level, regardless of whether or not this resultsin a second part of the substrate being processed by one or moresub-systems that have a lower performance level for that second area ofthe substrate.

As discussed above, an embodiment of the present invention is based on acomparison of the predicted performance criterion of the lithographysystem and the measured performance criterion of the lithography system.One or more performance criterion may be used.

By way of example, the performance criterion used may be one or moreselected from, but not limited to, the critical dimension of thepattern, the overlay error, the iso-dense bias, namely the differencebetween the critical dimension of sparsely arranged pattern features andthe critical dimension of densely arranged pattern features, and thedifference in the formation of differently-oriented elongate features inthe pattern. Such performance criterion are commonly used tocharacterize the performance of a pattern formed on a substrate. Theymay, for example, be determined for one or both of a test structure (ortarget as discussed above) and a structure that is part of a devicebeing formed on a substrate

An alternative or additional performance criterion of the formation of apattern on the substrate may be the productivity of each of theprocesses performed by the sub-systems, namely the time taken by eachsub-system to perform an associated process on the substrate and/or theoverall productivity of the lithography system. Each sub-system may havean expected time to perform each process and a deviation from this timemay be indicative of a problem with the process or a previous process.For example, during the exposure of a substrate using the scan mode, asdescribed above, the position of the substrate relative to theprojection beam of radiation may be adjusted to take into account thesurface variations of the substrate. As a result, if there are asignificant number of surface variations, the scan speed may need to bereduced, resulting in a longer time being required to expose thesubstrate. Therefore, an increased time being required for an exposureprocess may be indicative of a problem during a pre-exposure process,resulting in such surface variations, for example.

It should be appreciated that, regardless of the performance criterionconsidered, the lithography system may predict and measure theperformance criterion for a plurality of different areas on thesubstrate. Accordingly, the output of the comparator 35 may, forexample, be a distribution of the difference between the predicted andmeasured critical dimension of the pattern across the entirety of thesubstrate.

As discussed above, differences between the predicted and measuredperformance criterion may have a characteristic pattern from which itmay either be determined which of the sub-systems is not performing asexpected or the level of performance of one or more of the sub-systemsmay be determined. Such a characteristic pattern may be apparent fromthe differences between the predicted and the measured performancecriterion or from a combination of such differences and the actualperformance criterion as measured. In any case, the characteristicpattern may be apparent at a variety of levels.

Firstly, a characteristic pattern may be apparent across, for example,the exposure field of the exposure unit, for example resulting in largerdifferences between the predicted and measured performance criterion onone side of the exposure field than the other.

Secondly, the characteristic pattern may be apparent within each areacorresponding to a single device to be formed on the substrate. Suchareas, commonly referred to as “dies”, are typically exposed within asingle scan of the projection beam of radiation relative to thesubstrate. If a characteristic pattern is repeated within all such areason a substrate or an anomaly is identified at a characteristic locationwithin the die this may provide useful information regarding the sourceor nature of any problem. It will be appreciated that, as a result ofthe nature of the exposure process, some dies may be exposed as theexposure field scans across it in a first direction while other dies areexposed by a scan of the exposure field in the opposite direction. Ifthe characteristic pattern is only produced in dies in which the scanhas been performed in a given direction or the characteristic patternsare different for dies that are scanned in the opposite direction,further information regarding the source or nature of the error may beprovided.

Thirdly, a characteristic pattern may be produced across the entirety ofa given substrate, regardless of the arrangement of the exposures ofindividual dies on the substrate. Such a pattern may indicate, forexample, that the variation in the performance results from one or moreprocesses performed before or after the exposure of the substrate ratherthan as a result of a deviation from the expected performance in theexposure unit.

Fourthly, a characteristic pattern may be apparent from within a batchof substrates processed by the lithography system or by comparingdifferent batches of substrates processed by the lithography system. Forexample, a pattern may appear on some substrates within the batch butnot on others. In simple cases, it may be apparent that a pattern oferrors or deviations only occurs when a particular sub-system is used toperform a process on those substrates, clearly indicating that thedeviation in performance is derived from that sub-system. In othercases, the deviation may only occur where a particular combination oftwo or more sub-systems are used to perform processes on a substrate. Asa further example, it may be apparent that a particular deviation in theexpected performance consistently occurs at a given position within abatch, such as in the first or last substrate of the batch.

Finally, a pattern may be related to the time at which the substrate isprocessed by the lithography system. For example, some variations mayoccur at a particular time of day which may assist in identifying theircause. In particular, if the variation occurs at a time of day when aspecific event occurs within the production facility then it may beidentified that there is a link between that event and the deviation inperformance. As a further example, some variations may occur at aparticular time relative to a process being performed that is notdirectly directed to the operation of the lithography system. Forexample it may be identified that a variation regularly occurs after theapparatus has been sitting idle for a given length of time, has been inuse for a given length of time or in a process immediately after acalibration step is performed on a different but related sub-system,indicating that this is the cause of the deviation in performance.

EXAMPLES

The following are examples of characteristic patterns that may beidentified:

1. Substrates that are exposed while mounted to only one of twosubstrates tables within an exposure unit show an increased differencebetween the predicted and measured critical dimension and/or overlayerrors. This may indicate an error in the servo mechanism associatedwith that substrate table.

2. The first substrate within a batch shows an increased differencebetween the predicted and measured critical dimension. This may indicateerrors related to the stability of the system, such as the temperaturecontrol of one or more critical components.

3. The difference between the predicted and measured critical dimensionof the pattern varies across the exposure field of each die of thesubstrate. This may indicate a change in the illuminator uniformity.

4. The difference between the predicted and measured critical dimensionis significantly greater for dense lines than for isolated lines. Thismay indicate a variation in the radiation dose used for the exposure butis most likely not caused by an error in the focus control. Likewise, ifthe difference is greater for isolated lines, it is more likely that theproblem is caused by an error in the focus control.

5. The difference in the predicted and measured critical dimension isdifferent for horizontal and vertical lines. This may suggest an errorin the projection system or the illuminator system but not in thestage(s).

6. A difference in the predicted and measured critical dimension occursafter a long idle time of the lithography system. This suggests thatsome part of the lithography system has not had sufficient time to beprepared for continued operation and, for example, reached a stableoperating temperature.

7. The difference in predicted and measured critical dimension suddenlyincreases for a group of substrates in a second part of a batch ofsubstrates, for isolated lines in a particular area of the substrates.This sudden large focus error may suggest contamination on the substratetable.

8. The difference between the predicted and measured critical dimensionoccurs each time a particular patterning device (e.g., mask) is used.This suggests that there is a fault with the patterning device.

9. The difference between the predicted and measured critical dimensionvaries according to the distance from the center of the substrate. Thismay indicate a deviation from the expected performance of an etchersub-system.

10. The difference between the predicted and measured overlay errorvaries across the substrate with a pattern of twists and twirls. Thismay indicate, for example, a deviation from the expected performance ofa chemical mechanical polishing sub-system or a variation in thethickness of the resist applied to the substrate caused by radialmovement of the spin coat nozzle used to apply the resist in combinationwith acceleration of the substrate rotation.

As discussed above, the prediction of the performance criterion is basedon a model of the lithography system. The model of the system may, forexample, include one or more of the following operating conditions ofthe lithography system as model parameters: the measured performance ofthe projection system, such as through slit performance focal planeerrors, aberrations, stray radiation and numerical aperture; themeasured stage performance for the patterning device (e.g., mask) stageand the one or more substrate stages, including the servo error (movingaverage and moving standard deviation) for both horizontal and verticalmovements; the nature of the illuminator pupil, in particular thenominal shape, shape variations through the slit, the intensitydistribution, intensity variations through the slit and polarizationuniformity; the measured exposure dose; and known sensitivity for any ofthe above parameters (which may be derived with a commercial simulatorpackage based on the geometry of the image to be performed). Inaddition, the model for the resist, namely its composition and knownresponses (which is typically provided by the resist vendor) and thepatterning device critical dimension error (which is also often providedby the patterning device maker) may be included in the model.

It will be appreciated that the larger the number of operatingconditions taken into account in the model, the more accurately it maypredict the behavior of the lithography system. However, the greater thenumber of parameters, the more complex the model and the larger thenumber of calculations that may be required in order to predict theperformance criterion.

In an embodiment, the model is set up such that the performancecriterion prediction unit 27 can determine the predicted performancecriterion on-the-fly, namely during the operation of the lithographysystem such that it can use operating conditions data gathered duringthe process. It will be appreciated that the model may be configuredsuch that some of the calculations, involving operating conditions datathat is not measured during the operation of the lithography system (andmay not be updated frequently), are performed in advance.

Although specific reference may be made in this text to the use oflithographic apparatus or systems in the manufacture of ICs, it shouldbe understood that the lithographic apparatus and system describedherein may have other applications, such as the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, flat-panel displays, liquid-crystal displays (LCDs),thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“wafer” or “die” herein may be considered as synonymous with the moregeneral terms “substrate” or “target portion”, respectively. Thesubstrate referred to herein may be processed, before or after exposure,in for example a track (a tool that typically applies a layer of resistto a substrate and develops the exposed resist), a metrology tool and/oran inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in conjunction withother applications, for example imprint lithography, and where thecontext allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithography system comprising: a performance prediction unit, configured to predict a performance criterion of the formation of a pattern on a substrate by the lithography system based on an operating condition of the lithography system; an exposure unit, configured to expose a pattern of radiation on the substrate; a performance measurement unit, configured to measure a performance criterion of the formation of the pattern on the substrate by the lithography system, the measured performance criterion corresponding to the predicted performance criterion; a comparison unit, configured to compare the predicted performance criterion with the corresponding measured performance criterion; and a process controller configured to determine, based upon a difference in the predicted performance criterion and the corresponding measured performance criterion as determined by the comparison unit, which, if any, sub-system of the lithography system is not functioning in an expected manner, and configured to control the lithography system, wherein subsequent control of the lithography system is regulated by the difference in the predicted performance criterion and the corresponding measured performance criterion determined by the comparison unit.
 2. The system of claim 1, wherein if the difference between the predicted performance criterion and the corresponding measured performance criterion or the rate of change of the difference exceeds a respective threshold, at least one of the following occurs: a notification is issued, corrective action is taken, corrective action is scheduled, or the operation of at least part of the lithography system is suspended.
 3. The system of claim 1, comprising a plurality of sub-systems that are configured to perform the same process and a substrate routing memory configured to store information from which it can be determined which individual sub-system has performed or will perform each process for a substrate; and wherein the process controller is configured to use the substrate routing memory information in order to determine which specific, if any, sub-system is not functioning in an expected manner.
 4. The system of claim 3, wherein the process controller is configured such that, if the deviation of the performance of a sub-system from the expected manner, or the rate of change of the deviation, exceeds a respective threshold, the process controller changes the substrate routing memory information for one or more substrates yet to be processed such that a process that was to be performed by the sub-system is performed by one or more other sub-systems configured to perform the same process.
 5. The system of claim 3, wherein the process controller is configured to determine, based upon a difference in the predicted and measured performance criterion as determined by the comparison unit, a level of performance attainable by each sub-system and to set the substrate routing memory information for a substrate such that a process performed on the substrate is performed by one or more of the sub-systems necessary to achieve a desired performance of the pattern formation on the substrate.
 6. The system of claim 5, wherein the process controller is configured to determine the required performance of the pattern formation for the substrate from an analysis of features of the pattern to be formed.
 7. The system of claim 1, wherein the process controller is configured to initiate (i) a maintenance process, or (ii) a repair process, or (iii) a replacement process, or (iv) a calibration process, or (v) any combination of (i)-(iv) for a sub-system when it determines that deviation of performance of the sub-system from expected performance, or the rate of change of the deviation, exceeds a respective threshold.
 8. The system of claim 1, wherein the performance criterion includes (i) a critical dimension of the pattern, or (ii) an overlay error, or (iii) iso-dense bias, or (iv) a difference in the formation of differently-oriented elongate features in the pattern, or (v) any combination of (i)-(iv).
 9. The system of claim 1, wherein the performance criterion is predicted and measured for a plurality of different locations on the substrate.
 10. The system of claim 9, wherein a characteristic pattern of differences between the predicted and measured performance criterion is associated with variations in the performance of one or more sub-systems of the lithography system; and the process controller is configured to determine the performance of at least one of the sub-systems based on a recognition of the characteristic pattern.
 11. The system of claim 10, further comprising a sub-system performance memory configured to record the performance of the at least one sub-system determined by the process controller.
 12. The system of claim 10, wherein the characteristic pattern includes (i) variations across the exposure field, or (ii) variations across each device to be formed on a substrate, or (iii) variations across the substrate as a whole, or (iv) variations between substrates within a batch, or (v) variations related to the time and/or date that the substrate is processed, or (vi) any combination of (i)-(v).
 13. The system of claim 1, wherein the performance criterion includes a time taken by a sub-system of the lithography system to perform an associated process on the substrate.
 14. The system of claim 1, comprising at least one selected from the following: an illuminator configured to condition a projection beam of radiation, a patterning device configured to pattern the projection beam of radiation, a stage configured to hold the patterning device and control its position, a projection system configured to project the patterned beam of radiation onto a substrate, or a stage configured to hold the substrate and control its position, and wherein the operating condition of the lithography system used to determine the prediction of the performance criterion includes at least one selected from the following, as applicable: focal plane error, or aberration error, or stray radiation or numerical aperture of the projection system, or servo error of the stage configured to hold the patterning device, or servo error of the stage configured to hold the substrate, or nominal shape of an illuminator pupil of the illuminator, or shape variations through a slit of the illuminator pupil, or intensity variations through the slit of the illuminator pupil, or polarization uniformity across the illuminator pupil, or dose of radiation in each exposure, or nature of the resist applied to the substrate, or critical dimension error of the patterning device.
 15. A method of manufacturing a device using a lithography system comprising: predicting a performance criterion of the formation of a pattern on a substrate by the lithography system, based on an operating condition of the lithography system, wherein the operating condition of the lithography system used to determine the prediction of the performance criterion includes at least one selected from the following: focal plane error, or aberration error, or stray radiation, or numerical aperture of a projection system, or servo error of a stage configured to hold a patterning device, or servo error of a stage configured to hold a substrate, or nominal shape of an illumination pupil of an illuminator, or shape variations through a slit of an illumination pupil of an illuminator, or intensity variations through a slit of an illumination pupil of an illuminator, or polarization uniformity across an illumination pupil of an illuminator, or dose of radiation in each exposure, or nature of a resist applied to the substrate; exposing a pattern of radiation on the substrate; measuring a performance criterion of the formation of the pattern on the substrate by the lithography system, the measured performance criterion corresponding to the predicted performance criterion; and comparing the predicted performance criterion with the corresponding measured performance criterion, wherein the subsequent control of the lithography system is regulated by the difference in the predicted performance criterion and the corresponding measured performance criterion.
 16. A computer readable media containing program instructions to control a lithography system, the program instructions comprising: a performance prediction section, configured to predict a performance criterion of the formation of a pattern on a substrate by the lithography system based on an operating condition of the lithography system; a performance measurement section, configured to measure a performance criterion of the formation of the pattern on the substrate by the lithography system, the measured performance criterion corresponding to the predicted performance criterion; a comparison section, configured to compare the predicted performance criterion with the corresponding measured performance criterion; and a process control section, configured to control the lithography system, wherein the subsequent control of the lithography system by the process control section is regulated by the difference in the predicted performance criterion and the corresponding measured performance criterion determined by the comparison section, wherein if the rate of change of the difference exceeds a respective threshold, at least one of the following occurs: a notification is issued, corrective action is taken, corrective action is scheduled, or the operation of at least part of the lithography system is suspended. 